A blast furnace coke has hitherto been produced using, for example, a system schematically shown in FIG. 1. A coal, which has been previously pulverized and subjected to size control, is first transferred to a coal blending bin 1 and charged through a coal charging car 2 provided above a coke oven 3 into a coke oven chamber of the coke oven 3 of which the wall has been heated to 900 to 1100.degree. C. The temperature of the coal at the time of charge is 20 to 30.degree. C. Since the width of the coke oven chamber is about 400 mm and the thermal conductivity of the coal is very small, the average temperature rise rate of the coal within the coke oven chamber is as low as 3.degree. C./min. Therefore, in this conventional coke making process, a long period of time of 14 to 20 hr is required as the coking time. Thus, the conventional process posed problems of very low productivity and large energy consumption.
Further, in the above conventional blast furnace coke making process, a heavy caking coal has mainly been used for coke making due to the restriction of the quality of the blast furnace coke, making it difficult to expand the kinds of coal usable to coke making. In particular, a non-coking coal is more inexpensive than a caking coal, and the reserves thereof on earth are abundant. The use of such non-caking coal in a large amount leads to an improvement in profitability. However, blending of the non-caking coal as a coal for coke making in an amount of not less than 10% by weight unfavorably results in lowered coke strength.
Shortening the coking time by reducing the oven width is possible as means for improving the productivity. In this method, however, the amount of coal charge per chamber is reduced, making it impossible to improve the productivity of coke. On the other hand, increasing the coke oven length poses a problem of the difficulties of achieving even heating in the horizontal direction of the oven and a problem of the difficulties of discharging (pushing) coke after carbonization from the coke oven chamber. Such measures cannot markedly improve the productivity of coke.
Another method of shortening the coking time is to raise the temperature of a combustion flue provided on both sides of the coke oven chamber. However, due to a limitation on the material of bricks for the combustion chamber, there is a limit to the rise of the combustion flue temperature.
On the other hand, in order to shorten the coking time in the production of a blast furnace coke, a process has been developed wherein a coal for coke making is predried and preheated and then charged into a coke oven to shorten the coking time and to improve the charge density, enabling the quality of coke to be improved. For example, there is a precarbon method wherein a coal for coke making is preheated to about 200.degree. C. and then charged into a coke oven where the preheated coal is carbonized. In this connection, the preheating method and the method for carbonization in a coke oven are reported in Cokusu Noto (Coke Note) (Fuel Society of Japan, 1988), p. 134 and the like. In the precarbon method, the coal is preheated in order to improve the coking speed in the coke oven, that is, to improve the productivity of coke. The preheating temperature of the coal is low and about 180 to 230.degree. C. at the highest. An improvement in productivity of the coke is only 35% over the process not involving the step of preheating.
In order to markedly improve the productivity of coke and, at the same time, to diversify the coal usable for coke making, Japanese Unexamined Patent Publication (Kokai) No. 07-118661 proposes a process wherein a coal is preheated to 350 to 400.degree. C. and charged into a coke oven where the preheated coal is carbonized. In this method, however, the coal is merely heated to a high temperature, and it is difficult to markedly improve the caking property of the non-slightly-caking coal.
With this background, the development of a process wherein a coal is preheated to a high temperature and the caking property of the coal is improved by the preheating, making it possible to use a non-slightly-caking coal in a large proportion in the coal for blast furnace coke making and, at the same time, to markedly improve the productivity, has been desired in the art.